1. Field of the Invention
This invention relates to methods and devices for irradiating scattering media.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers in superscripts, e.g., x. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
Creating an optical focus inside scattering medium, such as biological tissue, has a great potential in various applications. However, optical scattering, as a dominant light matter interaction in the biological tissues, poses a very significant challenge. Recent developed wavefront shaping techniques have begun to address this1-4 by exploiting the deterministic and time-symmetric nature of scattering. Focusing of light through scattering media has been realized by iterative optimization method2,5, optical phase conjugation (OPC)6,7, or direct large scale measurement of the transmission matrix 8-10.
To focus light within a scattering medium requires an additional factor—a ‘guidestar’ mechanism to provide feedback or tagging, in order for the techniques to arrive at the right optical wavefront solution. Examples of guidestars include second harmonic generation11, fluorescence12,13 and kinetic14,15 targets. While these guidestars allow light focusing to their physical locations, these techniques fundamentally lack addressability if dense and randomly distributed guidestars are present.
Alternatively, ultrasound-assisted techniques, such as photoacoustic-guided10,16-18 and time-reversed ultrasonically-encoded (TRUE)19-21 optical focusing techniques, employ a focused ultrasound beam as a virtual guidestar that can confine the focus at a freely addressable position. While TRUE has a speed advantage over the photoacoustic approach, the TRUE guidestar is generally weak. In the lossless case, typically ˜1% of the probe light field that passes through the ultrasound focus is tagged22,23. Moreover, the resolution achieved is limited by the ultrasound focus size. Although more advanced TRUE techniques—iterative TRUE (iTRUE)24-26 and time reversal of variance-encoded light (TROVE)27, are able to break this resolution barrier, they achieve this at the expense of time. For practical biological applications with tight time constraints, efficient and fast techniques are highly desired.